1. Field of the Invention
The invention relates to an imbalance measurement machine.
2. Description of the Related Art
In the automotive industry, the manufacturers of motor vehicles set strict requirements for the quality of the vehicle tires used. Only those vehicle tires are permitted in which the imbalance of the vehicle tires, which can never be excluded, does not exceed a predetermined low value.
The manufacturers of vehicle tires must therefore test the vehicle tires that are produced as to whether or not they meet the predetermined low value. Vehicle tires with an overly great imbalance cannot be delivered to the motor vehicle manufacturers.
Measurement of the imbalance takes place with known imbalance measurement machines, which are structured in the manner of a spindle having a rotating part and a non-rotating part. The rotating part of the spindle is formed by a hollow shaft, which can be brought into rotation by means of a drive, and the non-rotating part of the spindle forms a hollow axle. The hollow shaft rotates relative to this hollow axle.
The vehicle tire that is to be measured for imbalance is attached to the shaft, which is not rotating yet, and then filled with air. For this purpose, a tubular air feed is provided centered relative to the hollow axle and the hollow shaft. The feed is connected with the shaft and rotates together with the shaft during the measurement of the imbalance.
At the end facing away from the vehicle tire, a rotational coupling is provided on the air feed, to which coupling an air hose for supplying the air required for filling the vehicle tire is connected. By means of the rotational coupling, it is possible to fill the vehicle tire with air when the shaft and the vehicle tire are already rotating. As a result, the duration of a complete measurement procedure can be kept low. Even while measuring the imbalance, the feed of air to the vehicle tire is maintained, so that it is possible to check the air pressure within the vehicle tire and to keep it constant. In many cases, electrical lines are therefore also connected with the rotational coupling, in addition to the air hose. These electrical lines are provided for an electrical circuit for regulation of the air pressure within the vehicle tire, in order to keep the air pressure constant while the imbalance is being measured.
Force transducers configured as sensors are situated between the hollow axle and the machine bed of the imbalance measurement machine. These sensors measure the forces between the hollow axle and the machine bed that occur during operation of the imbalance measurement machine. These forces are a measure of the imbalance.
The cause for the imbalance in a rotating vehicle tire is known to be attributable to an uneven mass distribution of the vehicle tire, i.e. the mass of the vehicle tire is not distributed with rotational symmetry. This uneven mass distribution leads to the result that the axis of rotation does not agree with the stable main axis of inertia of the vehicle tire, and that the axis of rotation is tilted in terms of its center of gravity.
When the vehicle tire is rotating, the imbalance brings about a bending moment on the axis of rotation, thereby causing circular vibrations displaced by 180° at the ends of the axis of rotation. The center of gravity of the vehicle tire remains at rest, in this connection, while the axis of rotation tumbles because of the vibrations. The forces that occur in this connection, which represent a measure of the imbalance, act on the force transducers and are measured by them.
In order to achieve the most precise possible measurement of the imbalance, only those forces that are exclusively generated by the imbalance of the vehicle tire should be recorded by the force transducers. In practice, however, it has been shown that this ideal case cannot generally be guaranteed. Because of the rotational coupling itself, and because of the air hose connected with the rotational coupling and because of the electrical lines additionally connected with the rotational coupling, if applicable, additional forces are introduced into the force transducers aside from the force that is caused by the imbalance. These forces influence and distort the measurement of the imbalance.
Because of these additional forces, the following can be observed in the known imbalance measurement machines. Different measurement values occur as a function of the angle at which the vehicle tire is attached to the shaft relative to a reference point to the shaft. If one represents these measurement values in Cartesian coordinates, at different angles distributed over 360°, one finds that the measurement values form a circle. The center point of this circle, however, is not the ideal center point of the coordinate system. Instead, the center point of the circle of the measured measurement values has a deviation from the ideal zero point of the coordinate system. The ideal zero point of the coordinate system occurs when the rotational coupling is not connected with the air feed, which can freely rotate together with the shaft, without any influence by outside forces. The deviation or zero point shift has different values for different tire types.
The deviation of the zero point occurs because in the measurement of the imbalance, not only the actual imbalance of the vehicle tire itself is measured. Instead, the measurement value is influenced by additional outside forces that are not caused by the vehicle tire but rather by the imbalance measurement machine itself. These outside forces mainly occur because the rotational coupling itself possesses an imbalance and stands in a fixed connection with the air hose and, if applicable, with electrical lines. The air feed therefore cannot rotate freely without any external influence. Instead, the rotational coupling has a true-running problem.
To counteract the true-running problem of the rotational coupling or to take the influence of the additional outside forces into consideration, in the known imbalance measurement machines the deviation of the zero point or the zero point shift is determined and documented for every vehicle tire type. This procedure is very complicated and takes about 30 minutes.
It is a further disadvantage that the zero point shift must be carried out separately for every new vehicle tire type. Furthermore, specific changes in the imbalance measurement machine require an immediate re-determination of all the correction values (zero point shifts). A further disadvantage is that the time-consuming determination of the correction values significantly restricts the availability of the imbalance measurement machine for the actual imbalance measurement.
If the zero point shift has been determined and is known for one vehicle tire type, measurement of the imbalance can take place using the known imbalance measurement machines. The correction value (zero point shift) assigned to the vehicle tire type in question is deducted from the resulting measurement value, and the correspondingly corrected measurement value approximately corresponds to the actual properties of the vehicle tire and reproduces the imbalance. It is easily evident that the determination of the imbalance in the known imbalance measurement machines is complicated and cost-intensive when using this method of procedure.